Meshing helical rotors

ABSTRACT

A conjugate pair of intermeshing rotors having helical lobes having helical crests and intervening grooves and adapted for rotation about parallel axes within a working space of a screw rotor machine. Each rotor has a tip circle, a pitch circle, and a root circle, one rotor of each pair being a female rotor such that the major portion of each lobe of said female rotor is located inside the pitch circle of the female rotor. The other rotor is a male rotor formed such that the major portion of each lobe of said male rotor is located outside said pitch circle of the male rotor. The lobes of one rotor follow the grooves of the other rotor to form a continuous sealing line between said pair of rotors, each of the lobes having a primary and secondary flank portion, wherein the primary flank portion of said lobes of the female rotor have a profile formed from at least one ellipse and the primary flank portion of the lobes of the male rotor have a profile formed from at least one ellipses.

[0001] This application claims the benefit of the filing date of provisional application 60/433,720, having a filing date of Dec. 16, 2003.

BACKGROUND OF THE INVENTION

[0002] Screw compressors and expanders are composed of meshing screw or helical rotors. As in the case of gears, screw rotors have pitch circles which represent locations of equal tangential velocity for conjugate pairs of rotors. The spiral grooves in the rotors are the locations of the volumes of gas which are trapped and in the case of compressors, compressed due to the coaction of a conjugate pair of rotors and an enclosing casing. Accordingly, the volumes of the spiral grooves are a major design consideration, their width, depth, length and number being important design variables. The shape of the cross section of the spiral grooves includes the variables of width and depth as well as the shape requirements for the driving/driven coaction between the conjugate pair of rotors. Additionally, the conjugate pair must meet the sealing requirements as the line contact advances along the rotor profile in the driving/driven coaction and as the rotor tips and end faces coact with the enclosing casing. This line contact follows the perimeters of the rotor profiles and is therefore at a varying tangential speed and has significant radial components. Additionally, the shape and cross section of the spiral grooves must meet requirements for ease of manufacture and cutting tool life. One problem associated with conventional screw rotor designs is that rotor profiles have generally been designed using a point generated and or circular profiles. These types of profiles are generally more difficult to machine as well as exposing the rotors to more significant impact with respect to seal line length, drive band contact stress, service life, and sensitivity to temperature fluctuations.

[0003] There exists a need therefore for a screw rotor profile for reducing seal line length, reducing contact stress, increasing service life, and exhibiting more flexibility to temperature fluctuation.

SUMMARY OF THE INVENTION

[0004] It is an object of this invention to increase the efficiency and longevity of a screw machine.

[0005] It is another object of this invention to provide a screw rotor profiles having reduced blow-hole area for improved efficiency.

[0006] It is yet another object of this invention to provide improved rotor tip curves which are less sensitive to tip clearance modification and which can be used for tapered rotors.

[0007] It is a further object of this invention to achieve the disclosed performance based objects while improving the manufacturability of the screw rotor profiles.

[0008] Another object is to reduce the contact stress between the male and the female rotors of a screw machine.

[0009] These objects, and others as will become apparent hereinafter, are accomplished by the present invention, a conjugate pair of intermeshing rotors having helical lobes having helical crests and intervening grooves and adapted for rotation about parallel axes within a working space of a screw rotor machine. Each rotor has a tip circle, a pitch circle, and a root circle, one rotor of each pair being a female rotor such that the major portion of each lobe of said female rotor is located inside the pitch circle of the female rotor. The other rotor is a male rotor formed such that the major portion of each lobe of said male rotor is located outside said pitch circle of the male rotor. The lobes of one rotor follow the grooves of the other rotor to form a continuous sealing line between said pair of rotors, each of the lobes having a primary and secondary flank portion, wherein the primary flank portion of said lobes of the female rotor have a profile formed from at least one ellipse and the primary flank portion of the lobes of the male rotor have a profile formed from at least one ellipses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawing wherein:

[0011]FIG. 1 is a simplified transverse section through the rotors of a screw machine employing the present invention.

[0012]FIG. 2 is a simplified view of the blow hole of the present invention as compared to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In the FIG. 1, the numeral 10 generally indicates a screw machine such as a screw compressor or an expander. Screw machine 10 has a casing 12 with overlapping bores 12-1 and 12-2 located therein. Female rotor 14 has a pitch circle, P_(F), and is located in bore 12-1. Male rotor 16 has a pitch circle, P_(M), and is located in bore 12-2. The axes indicated by points A and B are perpendicular to the plane of FIG. 1 and are parallel to each other and are separated by a distance equal to the sum of the radius, R_(F), of the pitch circle, P_(F), of female rotor 14 and the radius, R_(M), of the pitch circle, P_(M), of male rotor 16. The axis indicated by point A is the axis of rotation of female rotor 14 and the center of bore 12-1 whose diameter generally corresponds to the diameter of the tip circle, T_(F), of female rotor 14. Similarly, the axis indicated by point B is the axis of rotation of male rotor 16 and the center of bore 12-2 whose diameter generally corresponds to the diameter of the tip circle, T_(M), of male rotor 16.

[0014] As illustrated, female rotor 14 has six lands, 14-1, separated by six grooves, 14-2, while male rotor 16 has five lands, 16-1, separated by five grooves 16-2.

[0015] Accordingly, the rotational speed of rotor 16 will be 6/5 or 120% of that of rotor 14. Either the female rotor 14 or the male rotor 16 may be connected to a prime mover (not illustrated) and serve as the driving rotor. Other combinations of the number of female and male lands and grooves may also be used.

[0016] Generally referring to FIG. 1, the major portions of the rotor profile, that is leading flank or secondary flank D-B for both the male and female rotors, trailing or primary flank A-E for both the male and female rotors of the female and male rotors, 14 and 16, of the present invention are different ellipses or generated by different ellipses, with the tip or root portions being circular arcs. The ellipse allows for a continuously changing curved profile, as opposed to a fixed profile with circular curves, yielding a high radius at the drive band for reduced contact stress on the drive band, and a low radius near the rotor tip.

[0017] With reference to the FIG. 1, the male rotor tip segment A_(M)-B_(M) and the female rotor root segment A_(F)-B_(F) are each circular arcs having their centers at pitch point P_(M) and P_(F), respectively. The male rotor tip circle has a tangent contact point with segment A_(M)-B_(M) between points A_(M) and B_(M). The female rotor bottom circle with the root diameter of the female rotor has a tangent contact point with segment A_(F)-B_(F) between points A_(F) and B_(F). The male rotor tip segment A_(M)-B_(M) allow the male tip to have the traditional seal strips or to have the tapered rotors should they are required.

[0018] The leading flanks or secondary flanks of the male and female rotors have two segments. Convex segment B_(M)-C_(M) is part of an ellipse, with one of its axis overlapped with line B_(M)-P_(M) and having a common tangent at B_(M) with A_(M)-B_(M). Concave or concave-convex segment B_(F)-C_(F) is conjugally generated by ellipse B_(M)-C_(M). Segment B_(F)-C_(F) has a common tangent at B_(F) with circular arc A_(F)-B_(F). Points C_(M) and C_(F) may be just on or inside or outside the pitch circles of the male and female rotors respectively. Convex segment C_(F)-D_(1F) is part of an ellipse, with one of its axis overlapped with the radius of D_(F)-D_(1F) at point D_(F), and which has a common tangent at C_(F) with B_(F)-C_(F) and has a common tangent at D_(1F) with the circular arc D_(F)-D_(1F). Concave segment C_(M)-D_(1M) at the male rotor leading flank is conjugally generated by ellipse C_(F)-D_(1F). Segment C_(M)-D_(1M) has a common tangent at C_(M) with B_(M)-C_(M), and has a common tangent at D_(1M) with circular arc D_(M)-D_(1M).

[0019] The tip portion of the female rotor and the root portion of the male rotor have two segments. Segments D_(M)-D_(1M) and E_(M)-D_(M) are the two segments of the root portion of the male rotor, and segments D_(F)-D_(1F) and E_(F)-D_(F) are the two segments of the tip portion of the female rotor. D_(M)-D_(1M) is a concave circular arc with its center on the pitch circle of the male rotor, and D_(F)-D_(1F) is a convex circular arc with its center on the pitch circle of the female rotor. E_(M)-D_(M) is a convex circular arc with its center at the center A of the male rotor, and E_(F)-D_(F) is a convex circular arc with its center at the center B of the female rotor. Segment D_(M)-D_(1M) has a common tangent at D_(M) with E_(M)-D_(M), and segment D_(F)-D_(1F) has a common tangent at D_(F) with E_(F)-D_(F). The female rotor tip segments allow the female tip to have the traditional seal strips or to have the tapered rotors should they are required. The male root segments allow the male root to have the traditional seal grooves.

[0020] The trailing primary flanks of the male and female rotors have two segments. Segments A_(M)-F_(M) and F_(M)-E_(M) are the two segments of the trailing flank of the male rotor, and segments A_(F)-F_(F) and F_(F)-E_(F) are the two segments of the trailing flank of the female rotor. Convex segment A_(M)-F_(M) is part of an ellipse, with one of its axis overlapped with line A_(M)-P_(M) and having a common tangent at A_(M) with A_(M)-B_(M). Concave segment A_(F)-F_(F) is conjugally generated by ellipse A_(M)-F_(M). Segment A_(F)-F_(F) has a common tangent at A_(F) with circular arc A_(F)-B_(F). Point F_(F) is inside the pitch circle of the female rotor. Convex segment F_(F)-E_(F) is part of an ellipse, with one of its axis overlapped with the radius E_(F)-O₂ at point E_(F), and which has a common tangent at F_(F) with A_(F)-F_(F) and has a common tangent at E_(F) with the circular arc E_(F)-D_(F). Convex-concave segment F_(M)-E_(M) at the male rotor leading flank is conjugally generated by ellipse F_(F)-E_(F). Segment F_(M)-E_(M) has a common tangent at F_(M) with A_(M)-F_(M), and has a common tangent at E_(M) with circular arc E_(M)-D_(M).

[0021] As illustrated in FIG. 2, as a consequence of the above described profile, the area of the blow hole formed by the tip and leading flank sections of the meshing female 14 and male 16 rotors, is reduced by its shape being curved and narrower, in comparison to prior art blow holes formed by non-elliptical profiles, without reducing the blow hole height. By avoiding reduction in height, reasonable gas torque is maintained from the male to female rotor. As known in the art, the blow hole is a leakage channel which connects the leading and following cavities, and it reduces the total efficiency of helical screw compressor. This design, as described and as shown in FIG. 2, has the advantage of increasing performance of the compressor.

[0022] As a further consequence of the above described profile, the contact line length or seal line length between the male and female rotors are reduced. Since the seal line is one of the most important leakage channels inside a helical screw compressor, leading to reduction in both the total efficiency and volumetric efficiency, the reduction of the seal line length has the advantage of increasing performance of the compressor.

[0023] As an additional consequence of the above described profile, the drive band between the male and female rotors experience much lower contact stress. For a male drive screw compressor, if point B_(M) of ellipse segment B_(M)-C_(M) is located at the long axis of the ellipse, the radius at point C_(M) is much larger than the radius at point B_(M) due to the geometrical feature of an ellipse. The drive band is located on B-C and near point C, and the larger radius results in a larger relative radius, which results in lower contact stress. For a female drive screw compressor, the profile section design of segment F-E also gives the profile the ability to control the contact stress at the drive band.

[0024] Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims. 

What is claimed is:
 1. A conjugate pair of intermeshing rotors having helical lobes comprising helical crests and intervening grooves and adapted for rotation about parallel axes within a working space of a screw rotor machine, each rotor has a tip circle, a pitch circle, and a root circle, one rotor of each pair being a female rotor such that the major portion of each lobe of said female rotor is located inside said pitch circle of said female rotor, the other rotor being a male rotor formed such that the major portion of each lobe of said male rotor is located outside said pitch circle of said male rotor, the lobes of one rotor following the grooves of the other rotor to form a continuous sealing line between said pair of rotors, each of said lobes having a primary and secondary flank portion, wherein said primary flank portion of said lobes of said female rotor have a profile formed from at least one ellipse and said primary flank portion of said lobes of said male rotor have a profile formed from at least one ellipses.
 2. The conjugate pair of intermeshing rotors according to claim 1, wherein said primary flank portion of said lobes of said female rotor is formed by first and second tangent ellipses and said primary flank portion of said lobes of said male rotor is formed by first and second tangent ellipses. 